Power interruption or unstable power could cause abnormal system shutdown or data damage of computer equipments, or even cause damage of the equipment and result in huge loss. To prevent such a situation from occurring the power supply of conventional computer equipments usually is connected to an Uninterruptible Power Supply (UPS). The UPS provides power to allow the power supply to continuously maintain computer operation so that the computer equipments can get a time window to perform normal storing and machine shutdown. Refer to FIG. 1 for the structure of a conventional UPS and a coupling power supply. When city power is in a normal condition to provide power the power supply has an AC power input end 11 to receive AC power. The AC power passes through a front end commutation unit 12 and a power factor correction unit 13 and is transformed by a transformer 14 regulated by at least a switch 17 controlled by a pulse width control unit 15, and is output through a rear end commutation unit 16. The conventional UPS 60 also is connected to the city power which passes through an AC/DC converter 601, a charge circuit 604, a battery 605, a voltage boosting circuit 603 and a DC/AC converter 602. The input AC power is converted to DC power to charge the battery 605. In the event that the city power is not available the battery 605 outputs DC power which goes through voltage boosting and transformation to become AC power to be delivered to the AC power input end 11 of the power supply so that when blackout or unstable power occurs the power supply still can rely on the power provided by the UPS 60 to function for a selected time period. The conventional structure relies on the power supply to deliver output power. However, abrupt change or unstable of voltage often causes damage of the power supply. If the power supply is damaged because of abnormal input power, it cannot support and drive loads in a normal way even if the UPS has started operation to substitute the city power as the power source for the power supply. Hence there is still room for improvement on the conventional structure in terms of providing normal power supply output in the event of an abnormal condition happened to the power.